Subdomains

I have created an A record and I've pointed it to an IP. Now, something weird happens! I can see the subdomain, including files and folders, but my friends can't see the subdomain. And they only get this error :

ERROR 404
The server can not find the requested page : ...

I should mention that I asked them to ping the subdomain, and it seems the ping works well for them! but there is no directory listing or even opening index.html.